fix: DM avatar color and reactions in federation + explore/server discovery

- Fix DM welcome header avatar using home identity for correct gradient color
- Register DM channel IDs in channelOriginMap so federated DM operations
  (reactions, messages, typing) route to the correct instance
- Pass origin when creating DM channels from friends list and WS events
- Add server discovery/explore page with public server listings
- Add server visibility and description fields

packages/web/src/components/voice/PictureInPicture.tsx

@@ -351,7 +351,7 @@ export function PictureInPicture() {
               <Avatar
                 name={displayParticipant.username}
                 size={64}
-                userId={displayParticipant.userId}
+                userId={displayParticipant.homeUserId ?? displayParticipant.userId}
               />
               {speakingParticipantIds.has(displayParticipant.identity) && (
                 <div className="absolute -inset-1 rounded-full ring-2 ring-status-online animate-pulse" />